Conventionally, information reproducing apparatuses that use as a recording medium a magnetic disc such as a hard disc or an optical disc such as a compact disc (CD) have been used in computer external storage apparatuses, audio appliances, and the like.
In general, the higher the frequency of a signal reproduced from an optical disc or the like, the smaller its amplitude compared with in a low-frequency range (for example, 1 MHz or lower). That is, the reproduced signal tends to be weaker in a high-frequency range (for example, about 100 MHz or higher). This weakening of the reproduced signal occurs, for example in the case of an optical disc reproducing apparatus, due to, among other factors, the response characteristics of the optical pickup that detects in the form of an optical signal the information stored on an optical disc and the response characteristics of the circuit that converts the optical signal into an electrical signal. To make the weakened amplitude in the high-frequency range equal to the amplitude in the low-frequency range, it is conventionally common to raise the gain in the high-frequency range (hereinafter referred to as “boost”), thereby to achieve “equalization”.
Usually, the factor by which the high-frequency range is boosted (that is, the gain in the high-frequency range relative to the gain in the low-frequency range) in such equalization can be freely set by a control circuit or the like provided in an information reproducing apparatus. Thus, the factor by which the high-frequency range is boosted is so adjusted as to make the weakened amplitude of the reproduced signal in the high-frequency range equal to its amplitude in the low-frequency range. This contributes to an enhancement of the accuracy (that is, a reduction of read errors) with which the information recorded on a recording medium such as an optical disc is read.
The boost factor is adjusted, typically, by the use of an equi-ripple filter or the like. FIGS. 6 and 7 show the characteristics of an equi-ripple filter. As indicated by line 100 in FIG. 6, the gain in the high-frequency range is higher than in the low-frequency range. Moreover, as indicated by line 101 in FIG. 7, the group delay does not depend on frequency (that is, the phase-to-frequency characteristic is linear). Thus, between the input and output waveforms of the equi-ripple filter, there is no change in the group delay difference between the different frequency ranges.